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Among the children included in the study, those conceived during winter had a significantly greater risk of autism. The risk of having a child with an autism spectrum disorder grew progressively throughout the fall and winter to early spring, with children conceived in March having a 16 percent greater risk of later autism diagnoses, when compared with July conceptions.

The researchers says the finding, published in the journal Epidemiology, suggests environmental factors, for example, exposure to seasonal viruses like influenza, might play a role.

“The study finding was pronounced even after adjusting for factors such as maternal education, race /ethnicity, and the child’s year of conception,” says lead study author Ousseny Zerbo, a fifth-year doctoral student in the graduate group in epidemiology at the University of California, Davis.

Researchers included approximately 6.6 million birth records, or 91 percent of all births recorded during the study period. The children were followed until their sixth birthdays to determine whether they would develop autism.

The researchers identified which children were diagnosed with autism by matching birth records with those of children receiving services from the state Department of Developmental Services (DDS). Approximately 19,000 cases of autism were identified, with autism defined as “full syndrome” autism in the DDS records.

The study found that the overall risk of having a child with autism increased from month to month during the winter through the month of March. For the study, winter was considered the months of December, January and February. Each month was compared with July, with an 8 percent higher incidence in December, increasing to 16 percent higher in March.

Earlier studies’ findings about autism risk and month of conception or birth have had varied results. Some, such as ones conducted in Israel, Sweden, and Denmark, have found an increased risk of autism for children born in March. Studies conducted in Canada, Japan, the United States, and the United Kingdom identified an increased risk of autism for children born in the spring. However, these studies were far smaller, most having a few hundred cases of autism, when compared with the large number in California.

“Studies of seasonal variations can provide clues about some of the underlying causes of autism. Based on this study, it may be fruitful to pursue exposures that show similar seasonal patterns, such as infections and mild nutritional deficiencies,” says Irva Hertz-Picciotto, chief of the division of environmental and occupational health at UC Davis.

“However, it might be that conception is not the time of susceptibility. Rather, it could for instance be an exposure in the third month of pregnancy, or the second trimester, that is harmful,” says Hertz-Picciotto. “If so, we might need to look for exposures occurring a few months after conceptions that are at higher risk. For example, allergens that peak in the spring and early summer.”

The researchers say the study is a starting point for further inquiry. Other seasonal occurrences include potential exposures to pesticides, such as those used in the home to control insects in rainy or warm months, and those used in agricultural applications.

The work was funded by grants from the National Institute of Environmental Health Sciences of the National Institutes of Health.

More news from UC Davis: www.news.ucdavis.edu/